A decrease in size of the features of a pattern in a semiconductor wafer challenges resolution limits of optical inspection systems. A typical size for features of a pattern corresponding to electronic units is defined by design rules (DRs) of semiconductor wafers.
The shrinking design rules of semiconductor wafers lead to new challenges in optical inspection of the wafer. Pattern inspection techniques typically utilize a so-called bright field (BF) inspection mode.
Traditional BF inspection systems, which are based on resolved imaging of the pattern on a wafer, are limited in their spatial resolution due to diffraction limits of the optics in the imaging system. Using shorter wavelengths, such as the deep ultraviolet (DUV) spectral range, and increasing the numerical aperture (NA) of the imaging system may generally improve bright field imaging by reducing the diffraction limited spot size. However, when the typical dimension of the pattern feature becomes less than 90 nm, most of the features on the wafer (especially in array areas) cannot practically be resolved by suitable bright field imaging techniques.
On the other hand, the shrinking DRs result in noticeable diffractive effects from the pattern on the wafer. The diffraction orders are highly spread and the main diffraction lobes have high angular separation from the specular reflection of the pattern, i.e., zero order of diffraction. Such spatial separation of diffraction orders may be utilized for spatial filtering of the diffraction lobes. This can be utilized for blocking the diffraction lobes corresponding to large pitch components of the pattern and thus inspecting mainly for defects detectable as those causing light scattering in all directions. This technique presents dark field (DF) inspection/imaging and provides detection of defects in the pattern, which appear as bright spots on a dark background. DF inspection can be performed by collecting mainly scattered light, e.g., by blocking the specular reflection of light from the main pattern.
Efficiency of detection in dark field inspection mode is based on increases of the signal to noise ratio. Similarly to bright field inspection techniques, increasing the numerical aperture of the system operating in dark field mode enables more efficient collection of light and thus increases detection efficiency. The signal to noise ratio may also be increased by filtering light components of the diffraction orders providing a darker background and thus simplifying the detection of defects.
Various dark field imaging techniques have been developed aimed at improving the system performances.
For example, U.S. Pat. No. 6,686,602 describes an apparatus for spatial filtering including a Fourier lens, adapted to collect radiation emitted from a point and to separate the collected radiation into spatial components in a Fourier plane of the lens, and a programmable spatial filter, positioned at the Fourier plane. An image sensor is optically coupled to capture an image of the spatial components of the collected radiation in the Fourier plane, while the components are incident on the filter. A filter controller is coupled to receive and analyze the image captured by the image sensor and, responsive thereto, to control the spatial filter so as to block one or more of the spatial components.
U.S. Pat. No. 7,130,039 describes a compact and versatile multi-spot inspection imaging system employing an objective for focusing an array of radiation beams to a surface and a second reflective or refractive objective having a large numerical aperture for collecting scattered radiation from the array of illuminated spots. The scattered radiation from each illuminated spot is focused to a corresponding optical fiber channel so that information about a scattering may be conveyed to a corresponding detector in a remote detector array for processing. For patterned surface inspection, a cross-shaped filter is rotated along with the surface to reduce the effects of diffraction by Manhattan geometry. A spatial filter in the shape of an annular aperture may also be employed to reduce scattering from patterns such as arrays on the surface. In another embodiment, different portions of the same objective may be used for focusing the illumination beams onto the surface and for collecting the scattered radiation from the illuminated spots simultaneously. In another embodiment, a one-dimensional array of illumination beams is directed at an oblique angle to the surface to illuminate a line of illuminated spots at an angle to the plane of incidence. Radiation scattered from the spots are collected along directions perpendicular to the line of spots or in a double dark field configuration.